In methods of manufacturing the ferrite particles, popular methods includes mixing of predetermined amounts of raw materials for the ferrite particles, calcining, pulverizing, granulating followed by sintering. The calcining may be skipped depending on the conditions.
However, such a conventional method of manufacturing the ferrite particles has various problems. Specifically, in the sintering step for magnetization by ferritization reaction, as a tunnel kiln is typically used to sinter a raw material charged in a saggar, the shape of the particles tends to be irregular due to interaction among the particles. If the particle size of the ferrite particles should be smaller, the block-shape tends to be more significant after sintering to results irregularly-shaped particles by chipping in pulverizing. In addition, if the ferrite particles having a small particle size should be produced, the particles produced cannot have a preferable shape without vigorous pulverization. There is also a problem in that the production stability is poor because the required time for the sintering is about 12 hours including a period for elevating the temperature, a period for keeping the maximum temperature, and a period for decreasing the temperature, and the blocks formed after the sintering should be pulverized.
To solve such problems, new methods of manufacturing the ferrite particles have been proposed. For example, Patent Document 1 (Japanese Patent Laid-Open No. 62-50839) discloses a method of manufacturing a ferrite carrier in which a blended metal oxides as raw materials for ferrite formation is passed through a high-temperature flame environment to quickly ferritize the blended metal oxides.
In addition, Patent Document 2 (Japanese Patent Laid-Open No. 2008-216339) discloses a method of manufacturing a core material of an electrophotographic ferrite carrier under predetermined conditions in which a raw material powder with an average particle size of 20 to 50 μm is charged into a combustion flame with a carrier gas for the raw material powder and flame-spraying the powder for ferritizing the powder followed by rapid cooling for solidifying of the flame-sprayed particle in atmospheric air, and collecting the powder.
Furthermore, Patent Document 3 (Japanese Patent Laid-Open No. 2008-249855) discloses a resin-coated carrier for an electrographic developer including the ferrite carrier core material having a BET specific surface area of 900 to 5000 cm2/g and an apparent density of 2.30 to 2.80 g/cm3. Also for the method of manufacturing the ferrite carrier core material, it is considered preferable that a raw material powder is charged into combustion flame with a carrier gas for the raw material powder, flame-sprayed for ferritization of the powder followed by rapid cooling for solidifying in atmospheric air, and collected.
However, as the ratio between oxygen to the combustion gas is 3.5 or less in these production methods, the sintering may be difficult depending on the ferrite raw materials. In addition, these methods are not suitable for manufacturing a small-diameter ferrite particle and are not capable of forming the spherical ferrite particles uniformly.
Patent Document 4 (Japanese Patent Laid-Open No. 3-233464) discloses a method of manufacturing a carrier for an electrographic developer in which a carrier raw material is melted by a direct plasma process, high-frequency plasma process, or hybrid plasma process.
However, as such production method uses an expensive gas such as argon or helium, the method is very disadvantageous economically and impractical.
Fillers conventionally used are a pulverized product of a ferrite powder or calcined powder as a precursor of the ferrite powder having an irregular shape. Thus, gaps tend to be formed among particles in a coating or molding process, and uniform volume change does not occur during post-treatment (heating or sintering) carried out after molding, and it results, for example, generation of pores in the coated product or molded product or uneven sintering of the molded product. These phenomena tends to be more evident in the molded product or coated product of thinner and smaller.